The present invention relates to electric watthour meters and, more particularly, to voltage stators for watthour meters.
A conventional watthour meter employs a metallic disk supported for rotation by the interaction of eddy currents with magnetic fluxes produced by current and voltage stators disposed adjacent the disk. The rotation of the disk is retarded by a permanent magnet whose retarding torque is proportional to disk rotational speed. The torque producing rotation is proportional to the product of a load current times a voltage; that is, the torque is proportional to the power consumed by the load. the interaction of driving and retarding torques results in the disk rotational speed also being proportional to the power consumed by the load. Each rotation of the disk represents a predetermined quantum of energy consumption. The turns of the disk are accumulated to sum up the consumed energy in units such as, for example, watthours or kilowatt hours.
A conventional voltage stator, sometimes called a pot stator, employs a core built up of a stack of mutually-insulated, generally E-shaped, lamellae of magnetic material such as, for example steel. A coil of many turns of fine wire is disposed on the center leg of the core. Line voltage is applied to the coil to produce a magnetic flux in the core proportional to the line voltage. A magnetic flux appears in the two air gaps between the outside legs and the central leg. This magnetic flux interacts with the disk to induce eddy currents therein which, in turn, interact with a magnetic flux produced by the current stator.
A watthour meter is a precision measurement instrument required to operate with high accuracy over a large dynamic range of load power consumption. For example, an electric meter may be required to measure power consumption with an accuracy of better then one percent over a range of power consumption from about 10 percent to about 600 percent of test current with normal load voltage applied. Thus, accuracy must be maintained over a power ratio of about 60 or more.
Electric watthour meters provide for several adjustments to compensate for inevitable errors arising during manufacture. One adjustment compensates for errors in torque produced at a light load of 10 percent of test current and a power factor of 1.0 Such errors may arise from core nonlinearity; that is, at 10 percent of test amperes, the iron in the core may exhibit other than 10 percent of the flux produced when it receives test amperes. Other possible causes of light-load errors may arise due to friction or to mechanical or magnetic dissymmetries
A light load adjustment conventionally consists of a plate of conducting material, positioned in the space between the poles of the core and the disk, and means for adjusting its position in the space at right angles to the disk radius.
The effect of the light load adjustment is critically dependent on the magnitude of the main air gap flux (between the center leg of the core) and the disk. This magnitude, is, in turn, critically dependent on the magnitudes of the magnetic fluxes in the two auxiliary air gaps (between the two outside legs and the center leg of the core).
Theoretically, at least, the causes of light load error are constant for a given watthour meter. Thus, in theory, once the light load adjustment is properly completed, the compensation should be permanent without requiring further adjustment. In practice, however, changes in light load adjustment, once considered to be mysterious, do occur.
We have traced at least part of the change in light load adjustment to disturbance in the positions or spacing of the auxiliary air gaps. We observed that a watthour meter, properly adjusted for light load, frequently goes out of adjustment when it is subjected to shock or high levels of vibration.